God I love the "American = blonde" trope in anime/manga.
There are so many possibilities for it's existence and trying to figure it out is actually pretty fun.
It could be:
1. Mangaka's only interaction with the US is through movies and tv shows, where the number of blondes per capita is far higher, especially with actresses.
2. Mangaka don't really differentiate between western countries, causing countries that do have a large blonde population, such as Scandinavian countries, to be lumped in with the US in the mangaka's head when creating a character.
3. Mangaka don't really put that much thought into it and just roll with the established 'foreigner' look.